1. Field of the Invention
The present invention concerns a handover method and device for a cellular mobile radio system.
2. Description of the Prior Art
In a cellular mobile radio system (a system conforming to the GSM standard, for example), each cell has transceiver means for transmitting and receiving radio signals and a frame unit one function of which is to manage the radio channels in the associated cell; this combination is known as a base transceiver station, or base station for short. The transceiver means can send and receive radio signals on one frequency or on more frequencies if frequency hopping is used.
In time-division multiple access (TDMA) mobile radio systems, calls are transmitted on radio channels each characterized by a timeslot in the TDMA frame which is repeated periodically and a transmission frequency (or a plurality of transmission frequencies if frequency hopping is used). The timeslot in the TDMA frame corresponds to an on air timeslot for the mobile stations.
If a mobile station passes from one cell, called the old cell, to another cell, called the new cell, the continuity of the call in progress must be maintained; the availability of timeslots and the frequency or frequencies used for exchanging speech data change on moving from one cell to another. One method for providing this continuity in mobile radio systems is the intercellular transfer procedure usually referred to as "handover".
In the conventional way, the handover procedure entails activating a radio channel in the new cell, i.e. making it available to the mobile station for exchanging speech data, and then sending a handover instruction to the mobile station concerned so that it uses the new channel allocated to it. This is followed by a phase in which the mobile station is synchronized to the new base station.
After this synchronization phase the mobile station sends the new base station a message (called the SABM message in GSM terminology) initializing transmission context data (this data is known as LAPDm data; for more information on this topic see "The GSM System for Mobile Communications", M. MOULY and M. B. PAUTET, published by the authors, 1992, pages 261-262 and 268 through 277) relating to the new logical link (mobile station--new base station) created. This message is acknowledged by the new base station (in a message called the UA message in GSM terminology).
Between the time at which the mobile station receives the handover instruction and the time at which synchronization is completed transmission of speech data is interrupted with the result that either significant deterioration of speech quality must be tolerated during this period or speech extrapolation software must be used.
In mobile radio systems covering urban areas with very high traffic levels small cells are used (radius less than a kilometer, usually 200 m to 300 m). In these microcellular systems handover is extremely frequent because there is a high probability of a mobile station crossing more than one cell during the same call, given the small size of the cells.
The handover procedure as just described is lengthy: the time elapsing between reception of the handover instruction by the mobile station and completion of the synchronization phase is in the order of 100 ms to 200 ms. It is totally unacceptable for speech quality to deteriorate for such a long period and several times in succession.
In such cases it is therefore necessary to use speech extrapolation; this is not satisfactory, however, since if speech is extrapolated by more than 80 ms the user can perceive that what he is hearing is not what he should be hearing.
To solve this problem, patent EP-A-0 347 396 proposes to carry out handover in such a way that the call continues in the same timeslot and on the same frequency in the new cell.
This method presupposes the continuous availability of identical frequencies in a plurality of adjoining cells, or dynamic management of frequency allocation on the basis of interference. A method of this kind is complex to implement in systems conforming to the GSM standard, in particular because of the interference that is likely to result.
Also, use of this method makes it necessary to verify two criteria: the availability of the timeslot in the new cell, and the availability of the frequency in the new cell. Most of the time there is little chance of these two criteria being satisfied simultaneously, with the result that this method can be used only in very rare cases.
One object of the present invention is to provide a handover method in which speech is interrupted for a shorter time than in the conventional methods.
Another object of the present invention is to provide a handover method which is simple to implement in systems conforming to the GSM standard and succeeds in a large proportion of cases in which handover is needed.